Short-circuit releases are used in power circuit-breakers for switching and protecting motors and other loads. These short-circuit releases are designed as electromagnetic releases, which essentially include a coil winding, a coil element, an armature, a pole, a plunger, a restraining spring and a yoke. The armature is attracted at a specific rated current of the power circuit-breaker, for instance at twelve times the nominal current in the case of motor protection or at nineteen times the nominal current in the case of transelement protection. The armature's motion therein acts upon a breaker mechanism and on a moveable contact member in order to open the contacts. The relevant standard therein specifies that the operating current can vary by at most +/−20%.
Because of the requisite larger supporting cross-sections, smaller numbers of turns, wider tolerances for the coil and wrap wire, and the greater magnetic-field inhomogeneity associated therewith, the difficulty with relatively wide adjustment ranges lies in positioning the coil winding with respect to the air gap between the armature and pole sufficiently accurately to enable the operating limits to conform to the relevant standard. There is also the problem of fixing the coil winding in its determined position with respect to the air gap so that the coil winding will not become displaced in the direction of the center of gravity of the iron at the rated current or when the short-circuit current is high, or if the coil contracts and becomes deformed with the result that the operating limits will then no longer be adhered to.
For higher breaking capacities, the coils are produced having the winding turns resting on them so that the coil cannot contract and become deformed in the event of high short-circuit currents. Owing to the use of uniform coil elements for the respective structural size and the fact that they are designed for the geometrically largest coil winding, there is often a gap between the coil element flange or yoke and the last turn of the coil winding. In order to fix the coil winding once it has been positioned accurately with respect to the air gap between the armature and pole, one of the coil winding's ends is bonded to the coil-element flange or yoke and the other is welded to a terminal.
In the case of power circuit-breakers having a high breaking capacity, for example up to 100 kA for a nominal current of 80 A, the current release module must be completely redimensioned in order to control the thermal stress and to achieve the release behavior required therefor. The power rating in the case of the short circuit release increases here on account of the increased contact load. It should also be noted that the power loss of an 80-A release is not permitted to be greater than that of the 50-A release in order to prevent an increase in the temperature. This means that in accordance with the overall design of the short-circuit release the magnetic circuit must be designed more efficiently.
With the previously known 80-A devices, a shock armature system was dispensed with. A different force level was used to configure the short-circuit release. The release only has to release the breaker mechanism and not impinge on the contact bridge. This allows for a much more unfavorable magnetic system. The disadvantage here is that a contact welding can be prevented with more difficulty.